1. Field of the Invention
This invention relates to a filter medium which permits an increased amount of filtration.
2. Description of the Prior Art
Conventional processes for filtering molten aluminum include a method wherein a soft medium such as a glass fiber or stainless steel screen is used, a method wherein a fixed bed of alumina is utilized, and a method wherein a rigid medium is used. The last of these methods uses an apparatus comprising a filter tank equipped with an inlet and an outlet for molten aluminum and a heating lid equipped with a heat generating element and a thermocouple, and involves passing molten aluminum from the outside to the inside of a tubular hard medium by the difference in the surface level of incoming molten aluminum and outgoing molten aluminum. This method is superior to the other methods with respect to the substantial effects of the filtration and its adaptability to field work, etc.
In general, a filter medium such as a hard filter medium should possess the following properties.
1. It should have a number of air-permeable pores or proper size and of high uniformity.
2. It should be resistant to corrosion by the material to be filtered.
3. It should have thermal resistance and spalling resistance.
4. The aggregate particles should be firmly bonded to one another and not cause filtration failure.
5. It should have mechanical strength higher than a certain level.
It has previously been known to produce a filter medium such as a hard medium using an aggregate material and a fritted vitreous binder. For example, a filter medium including a vitreous bonding material as disclosed in U.S. Pat. No. 3,524,548 has superior corrosion resistence to molten aluminum and permits stable filtration to be conducted.
However, with the vitreous bonding material used in this U.S. Patent, it is difficult to control the structure of the filter medium comprising an aggregate material, the bonding material and air-permeable pores. Accordingly, the filtration efficiency of the filter medium cannot be increased by providing a number of the air-permeable pores of a constant size.
Where the bonding material is vitreous, the following defects are encountered.
1. Since the viscosity of the bonding material gradually decreases with a rise in temperature during calcining, it is difficult to obtain increased distances between aggregate particles and to lengthen the linking bridges of the bonding material.
2. If a combustible substance is used with such a bonding material in order to increase the proportion of the air-permeable pores in the filter medium, the combustible substance carbonized within the bonding material is occluded in the vitreous substance and caused a reduction in mechanical strength and often leads to filter failure.
Accordingly, when a vitreous substance is used as the bonding material, there can be obtained only a filter medium which has a structure determined only by the nature of the aggregate particles. While the size of the pores can be controlled by varying the particle size of the aggregate, it is difficult to control the proportion of the pores in the filter medium.